U.S. Pat. Nos. 5,229,464 and 5,382,604 describe conjugated diene block copolymers which have at least one block each of two different polymerized conjugated dienes. In their preferred embodiments, the conjugated dienes are isoprene and butadiene. The patents describe methods for making such polymers and then hydrogenating them in order to as completely as possible hydrogenate the polybutadiene blocks while leaving a certain amount of residual unsaturation in the polyisoprene blocks. This residual unsaturation may then be reacted to attach epoxy groups to the polymer or some other functionality. Generally, the hydrogenation process described utilizes a nickel-aluminum catalyst as described in U.S. Pat. No. 4,879,349 but other Group VIII metals and titanium catalysts have been used as well as described in U.S. Pat. No. 5,039,755.
Polybutadiene is known to be relatively easy to hydrogenate whereas polyisoprene is more difficult to hydrogenate. The rates of hydrogenation of both of the main microstructure configurations of polybutadiene, 1,2-butadiene and 1,4-butadiene, are higher than the rate of hydrogenation of the main microstructure configuration in polyisoprene, 1,4-isoprene. The rate of hydrogenation of 1,4-butadiene is noticeably faster than that of 1,4-isoprene and the rate of hydrogenation of 1,2-butadiene is noticeably faster than that of 1,4-butadiene. Thus it can be seen that the hydrogenation of the polyisoprene blocks in polybutadiene-polyisoprene block copolymers can be a rate limiting step in the overall process for the production of hydrogenated polybutadiene-polyisoprene block copolymers.
The current mode of hydrogenating such block copolymers involves two stages. In the first stage, the focus is on hydrogenating the polybutadiene blocks as completely as possible, for example to a residual unsaturation of 0.3 meq/g, which for a 4800 number average molecular weight butadiene block is equivalent to a total butadiene double bond conversion of 95% as shown in Example 3 below. Residual unsaturation in this sense means the milliequivalents of unsaturated double bonds per gram of polymer which are left in the polymer block after the hydrogenation step. Residual unsaturation is thus defined once the total conversion of double bonds as a percentage is known and the molecular weight of the polymer or the polymer block is known or chosen. In the second stage, more stringent conditions or more catalyst are utilized to hydrogenate the polyisoprene blocks to the desired level.
The biggest problem with this approach is that the total hydrogenation part of the process can become the rate-limiting step in the process. If a way could be found to reduce the residence time or cycle time of the hydrogenation part of the process, the overall cost of producing such polymers could be significantly decreased. Residence time is the average time the polymer cement spends in a continuous hydrogenation in a batch reactor. Cycle time is the complete time taken to complete the hydrogenation in a batch reactor. These terms are used interchangeably herein.
One obvious way to attempt to speed up the hydrogenation of the polyisoprene blocks is to add more hydrogenation catalyst. Obviously, this increases the cost as the hydrogenation catalyst itself is expensive. Unfortunately, the variability in catalyst activity can make it difficult to target a particular polyisoprene block residual unsaturation. Adding more catalyst to increase the rate of hydrogenation also increases the risk of overshooting the target residual unsaturation. If a narrow specification is required for residual unsaturation, this method is not very effective.
It can be seen that it would be highly advantageous to find a method which would allow one to produce polybutadiene-polyisoprene block copolymers in which the polybutadiene block is almost completely hydrogenated while leaving a specified amount of residual unsaturation within a narrow specification range in the polyisoprene block and doing so at an overall rate that minimizes the residence time of the hydrogenation step. This would allow a cost effective method for producing such hydrogenated polybutadiene-polyisoprene block copolymers.